1. Field of the Invention
The present invention relates to a high-frequency switch for intermitting a high-frequency signal, and more particularly to a high-frequency switch having high isolation characteristics.
Also, the present invention relates to an adjustable high-frequency power amplifier which is capable of adjusting its output power.
Further, the present invention relates to a monolithic microwave integrated circuit (MMIC) in which the high-frequency switch or the adjustable high-frequency power amplifier described above is formed.
2. Description of the Related Art
In recent years, mobile communications such as cellular phone has been shifting from analog mobile communication systems to digital mobile communication systems by virtue of the rapid progress of LSI technology.
One of the digital mobile communication systems is the PDC (Personal Digital Cellular) system. The PDC system adopts the TDMA (Time Division Multiple Access) system, and the portable terminal can perform the packet communication of, at most, 28.8 kbps. The portable terminal of the PDC system performs the control of varying its output power within a range of about 30 dB in accordance with the distance thereof from a base station.
FIG. 1 shows an example of a circuit which is used in a power amplifier for transmission in the portable terminal of the PDC system. The power amplifier 1 includes a bipolar transistor 2 which has an emitter grounded. The transistor 2 has a finger number corresponding to the maximum output power of the portable terminal. The base of the transistor 2 receives an input signal IN of high frequency through an input matching circuit 3. The collector of the transistor 2 outputs an output signal OUT generated by amplifying the input signal IN, through an output matching circuit 4. The base of the transistor 2 is supplied with a base bias through an inductor 5 which features sufficiently high impedance at the frequency to be used. The collector of the transistor 2 is supplied with a supply voltage VCC through an inductor 6 which features a sufficiently high impedance at the frequency to be used.
The power amplifier 1 described above receives from another circuit of the portable terminal the input signal IN (voice or data) of the power in accordance with the distance thereof from the base station. The transistor 2 amplifies the input signal IN, and sends the amplified signal as the output signal OUT toward the base station.
Meanwhile, it is known that, in general, the portable terminal of the PDC system or the like often uses an output power level which is about 10 dB lower than the maximum output power. The circuit shown in FIG. 1 is used the bipolar transistor 2 of the finger number corresponding to the maximum output power. Therefore, as the output power is lower, a wasteful steady current not contributing to the amplification flows more through the transistor 2. As in the case when the output power level is often used described above, a considerable amount of steady current has flowed through the transistor 2.
On the other hand, the W-CDMA (Wideband Code Division Multiple Access) system has been proposed as a new digital mobile communication system. A portable terminal of the W-CDMA system diffuses a transmission signal by using the diffusion code, and outputs diffused data to the base station. Besides, the W-CDMA system is specified so as to change its output power, depending upon voice and data, and the portable terminal of this system must control the output power in a wide range more than 70 dB.
In a case where the power amplifier 1 shown in FIG. 1 is applied as a power amplifier for transmission in the portable terminal of the W-CDMA system, the finger number of the transistor 2 is determined in adaptation to the maximum output power similarly to that in the PDC system. However, in the W-CDMA system, the control range of the output power is predetermined more than 70 dB. Therefore, as the output power is lower, a wasteful steady current not contributing to the amplification flows more through the transistor 2. This steady current is greater than the steady current in the power amplifier of the portable terminal of the PDC system described above. The power amplifier is one of circuits which consume electric power most in the portable terminal. Accordingly, in the case where the power amplifier 1 shown in FIG. 1 is applied to the portable terminal of the W-CDMA system, there have been the problems that the power consumption of the portable terminal increases, that the quantity of heat generation thereof enlarges, and that the talking time is obliged to shorten.
In order to reduce the power consumption of the portable terminal of the W-CDMA system, the inventor has considered that two power amplifiers are combined so as to operate either or both of them, thereby to vary the output power of the portable terminal. In case of configuring such an adjustable power amplifier, however, the portable terminal needs to be provided further with a change-over circuit and matching circuits for high-frequency signals. As a result, there have been the problems that there needs to be a number of components and the portable terminal cannot be made in small size.
As a technique for making the size of the portable terminal small, it is considered that the power amplifier or the like are implemented on an MMIC (Monolithic Microwave Integrated Circuit), thereby making the total number of components smaller. However, there has not been proposed such a circuit technology using bipolar transistors wherein a high-frequency switch, which has high isolation characteristics in an off-control mode and has little influence on the input side corresponding to the change of a load in an on-control mode, is formed on the MMIC. Therefore, it has been impossible to form the adjustable power amplifier on a single chip as the MMIC.